Ninth Grade · Science · week 23

Protein Synthesis: Transcription and Translation

Trace the flow of genetic information from DNA through mRNA to a polypeptide, using transcription and translation.

The central dogma of molecular biology describes information flowing from DNA to RNA to protein. Transcription happens first, in the nucleus: RNA polymerase unwinds a section of DNA and builds a complementary messenger RNA (mRNA) strand using one DNA strand as a template. RNA differs from DNA in three ways worth stating plainly: RNA is single-stranded rather than double-stranded, it uses the sugar ribose instead of deoxyribose, and it uses the base uracil in place of thymine. The finished mRNA is processed and then exits the nucleus to reach a ribosome in the cytoplasm.

Translation then reads that mRNA three bases at a time -- each three-base unit is called a codon. The genetic code matches each of the 64 possible codons to one of twenty amino acids, or to a stop signal, and this code is nearly universal across all known life, itself a piece of evidence for shared ancestry that Unit 9 returns to. Transfer RNA (tRNA) molecules, each carrying one specific amino acid and a matching three-base anticodon, pair up with each codon in turn at the ribosome; the ribosome links each arriving amino acid to the growing chain with a peptide bond (a direct callback to Unit 2), continuing until a stop codon is reached. The finished polypeptide then folds into a functional protein, exactly as Unit 2 described.

📋 For the teacher
Teach it (10–15 min):
  • A "codon cracking" activity works well: write out a short made-up mRNA strand (12 to 15 bases) and have your student translate it codon by codon using a standard genetic code chart, then compare their translated peptide to a classmate's or sibling's translation of the same strand with a single base changed, to see firsthand how much, or how little, one change can affect the resulting protein.
  • The near-universal genetic code, where the same codons specify the same amino acids in nearly every living thing, is worth pausing on -- it is one of the pieces of evidence for shared ancestry that Unit 9 will return to.
  • Walk through the flow DNA to mRNA to protein slowly, with your student narrating each step in their own words rather than just naming vocabulary -- narrating the process is what proves real understanding here.
  • If your student struggles with the transcription step's base-pairing, revisit week 21's complementary-strand practice -- transcription uses the exact same base-pairing logic, just producing RNA instead of a second DNA strand.
Talk about it:
  • Why does it make sense for a cell to make a temporary mRNA copy of a gene rather than sending the original DNA out of the nucleus to the ribosome?
  • What does it suggest about the history of life that nearly all living things use the same genetic code?
  • Why might a single base change early in an mRNA sequence sometimes matter enormously, and other times barely matter at all?

Mastery looks like: They can transcribe a DNA template into mRNA and translate that mRNA into a short amino acid sequence using a codon chart.

Common stumbles: Forgetting to swap thymine for uracil when transcribing; reading codons out of the correct three-base frame, since shifting by even one base changes every downstream codon.

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